Control circuit, control method and led driving circuit thereof

ABSTRACT

A control circuit for an LED driving circuit having a rectifier and a power transistor for driving an LED load, can include: a control signal regulation circuit configured to control a driving voltage of the power transistor to vary with a rectifier output voltage to control the variation of a current flowing through the power transistor to be consistent with that of the rectifier output voltage to decrease a power loss of the power transistor; and the control signal regulation circuit being configured to control the driving voltage of the power transistor to vary with the rectifier output voltage to control the variation of the current flowing through the power transistor to be opposite to that of the rectifier output voltage to improve a power factor of the LED driving circuit.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201610286012.4, filed on Apr. 29, 2016, and of Chinese PatentApplication No. 201610808778.4, filed on Sep. 6, 2016, both of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of powerelectronics, and more particularly to LED control circuits and methods.

BACKGROUND

With increasingly use of light-emitting diode (LED) lights, multi-stageswitching converters are typically used in order to adjust thebrightness of the light sources. In addition, each stage may deal withthe power in total, which can increase product costs of the LED drivers.In order to reduce the costs, some conventional techniques divide thesecondary winding at the output side of a flyback converter into twogroups, and a DC-DC converter of the second stage may be connected toonly one of the two groups. In accordance one LED luminance system, inorder to achieve current balance of each LED branch circuit, a linearregulator (LDO) can be coupled with the LED branch circuit to regulatethe LED driving current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an example linear LED drivingcircuit.

FIG. 2 is a schematic block diagram of a first example linear LEDdriving circuit, in accordance with embodiments of the presentinvention.

FIG. 3 is a schematic block diagram of a second example linear LEDdriving circuit, in accordance with embodiments of the presentinvention.

FIG. 4 is a waveform diagram of example operation of the linear LEDdriving circuit of FIG. 3, in accordance with the embodiments of thepresent invention.

FIG. 5 is a schematic block diagram of a third example linear LEDdriving circuit, in accordance with embodiments of the presentinvention.

FIG. 6 is a waveform diagram of example operation of the linear LEDdriving circuit of FIG. 5, in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Referring now to FIG. 1, shown is a schematic block diagram of anexample linear LED driving circuit. In this example, an input alternatevoltage can be rectified by rectifier bridge circuit DB1, in order togenerate a direct voltage signal across electrolytic capacitor EC1 to beprovided to the light-emitting diode (LED) load and power transistor Q,for example, voltage signal Ref can represent an average value of an LEDcurrent. The linearity of power transistor Q can be controlled by anerror between voltage signal Ref and a sensing voltage signal that isgenerated by sensing a current of sensing resistor RSEN. In this way,the LED current may be controlled to be constant. However, because thecurrent flowing through power transistor Q may be consistent with (e.g.,the same as) that of the LED load, power losses generated by sensingresistor RSEN and the on-resistance of power transistor Q may result indecreased efficiency.

In one embodiment, a control circuit for an LED driving circuit having arectifier and a power transistor for driving an LED load, can include:(i) a control signal regulation circuit configured to control a drivingvoltage of the power transistor to vary with a rectifier output voltageto control the variation of a current flowing through the powertransistor to be consistent with that of the rectifier output voltage todecrease a power loss of the power transistor; and (ii) the controlsignal regulation circuit being configured to control the drivingvoltage of the power transistor to vary with the rectifier outputvoltage to control the variation of the current flowing through thepower transistor to be opposite to that of the rectifier output voltageto improve a power factor of the LED driving circuit.

Referring now to FIG. 2, shown is a schematic block diagram of a firstexample linear LED driving circuit, in accordance with embodiments ofthe present invention. In this particular example, rectifier bridgecircuit DB1 can convert the received alternating voltage ACIN to busvoltage VBUS, which may be configured as a sinusoidal half wave directvoltage signal. Linear LED driving circuit 1 can be coupled betweenrectifier bridge circuit DB1 and the LED load. LED driving circuit 1 caninclude output capacitor Co in parallel with the LED load and powertransistor Q, where power transistor Q can be coupled in series with theLED load. The common node between output capacitor Co and the LED load(e.g., the positive terminal of output capacitor Co) can connect to thepositive output terminal of rectifier bridge circuit DB1 in order toreceive bus voltage VBUS. Power transistor Q can connect between thecathode of the LED load and the negative output terminal of rectifierbridge circuit DB1.

Alternatively, power transistor Q can be coupled between the positiveoutput terminal of rectifier bridge DB1 and the anode electrode of LEDload, or in any other suitable series connection with the LED load.Power transistor Q can be controlled by control circuit 2. Gate drivingvoltage GATE can be controlled by control circuit 2 in order to varywith bus voltage VBUS. Therefore, the variation of current flowingthrough power transistor Q can be the opposite to the variation of busvoltage VBUS, in order to decrease power losses of power transistor Q.Alternatively, an average current flowing through power transistor Q canbe controlled to be consistent with (e.g., the same as) an expectedvalue by simultaneously driving voltage GATE of control circuit 2.

Control circuit 2 can include feedback comparison circuit 21 and controlsignal regulation circuit 22. Feedback comparison circuit 21 cangenerate compensation voltage signal Vc in accordance with samplingvoltage signal Vs that represents the current flowing through powertransistor Q, and reference voltage signal VREF. Compensation voltagesignal Vc may represent error information between sampling voltagesignal Vs and reference voltage signal VREF. Also, reference voltagesignal VREF can represent an expected value of a driving current of LEDload. Sampling voltage signal Vs may represent an average value of thecurrent flowing through the LED load. For example, sampling voltagesignal Vs can be obtained through sampling resistor RSEN coupled inseries with the LED load and power transistor Q.

Control signal regulation circuit 22 can generate driving voltage signalGATE in accordance with compensation voltage signal Vc and bus voltageVBUS. In one example, driving voltage signal GATE can be generated inaccordance with compensation voltage signal Vc and a voltage signal thatrepresents bus voltage VBUS by control signal regulation circuit 22 tocontrol the current flowing through power transistor Q. In anotherexample, driving voltage signal GATE can be generated in accordance withanother voltage signal that represents compensation voltage signal Vcand the voltage signal that represents bus voltage VBUS.

In accordance with FIG. 2, the amplitude of the voltage signal thatrepresents bus voltage VBUS (e.g., V1) can be in direct proportion withbus voltage VBUS, and the polarity of voltage signal V1 and bus voltageVBUS may be opposite to each other. The variation of the current flowingthrough power transistor Q can be opposite to that of bus voltage VBUSbecause driving voltage signal GATE can be generated in accordance withvoltage signal V1. In the first half period of half power frequencyperiod of bus voltage VBUS, current flowing through power transistor Qmay be gradually decreased while bus voltage VBUS is graduallyincreased. In the last half period of half power frequency period of busvoltage VBUS, current flowing through power transistor Q can begradually increased while bus voltage VBUS is gradually decreased.Therefore, the driving current of the LED load may be regulated inaccordance with the variation of bus voltage VBUS. The average value ofthe current flowing through the LED load can be controlled to meet therequirement, and when bus voltage VBUS is greater than a givenrequirement, the current flowing through power transistor Q may bedecreased in order to decrease power losses of power transistor Q toimprove efficiency.

Referring now to FIG. 3, shown is a schematic block diagram of a secondexample linear LED driving circuit, in accordance with embodiments ofthe present invention. Also referring to FIG. 4, shown is a waveformdiagram of example operation of the linear LED driving circuit of FIG.3, in accordance with the embodiments of the present invention. In theparticular example of FIG. 3, feedback compensation circuit 21 caninclude sensing resistor RSEN, error amplifier EA2, and compensationcapacitor C_(COMP). Sensing resistor RSEN can be used sample the currentflowing through power transistor Q, and to generate sensing voltagesignal Vs that represents the current flowing through power transistorQ. Error amplifier EA2 can generate an error signal in accordance withsensing voltage signal Vs at its inverting input terminal and referencevoltage signal VREF at its non-inverting input terminal. In addition,the error signal can be compensated by compensation capacitor C_(COMP)in order to generate compensation voltage signal Vc. Compensationcapacitor C_(COMP) can be configured as a bulk capacitor to average theerror signal, and to substantially eliminate power frequency ripples.

As shown in FIG. 3, control signal regulation circuit 22 can include avoltage divider resistance network (e.g., resistors R1 and R2 connectedin series) and a first voltage conversion circuit. For example, thefirst voltage conversion circuit can be configured as voltage-controlledvoltage source El. The voltage divider resistance network can connectbetween the two output terminals of rectifier bridge DB1, and may beconfigured to generate voltage divider signal VBUS1 at the common nodebetween resistors R1 and R2 in accordance with bus voltage VBUS.Voltage-controlled voltage source E1 can convert voltage divider signalVBUS1 to voltage signal V1 with an opposite variation to bus voltageVBUS. For example, the amplitude of voltage signal V1 can be in directproportion with that of bus voltage VBUS, and the polarity of voltagesignal V1 may be opposite to that of bus voltage VBUS. The waveform ofvoltage signal V1 may be as shown in FIG. 4, and in one half of thepower frequency period (e.g., from t1 to t2), the variation of voltagesignal V1 may be opposite to that of bus voltage VBUS.

One skilled in the art will recognize that the configuration of thefirst voltage conversion circuit described above is not limited tovoltage-controlled voltage source, and other suitable circuitconfigurations can also be applied (e.g., a circuit including proportionconversion circuit and a polarity conversion circuit). In addition,control signal regulation circuit 22 can also include a second voltageconversion circuit, which may be configured as voltage-controlledvoltage source E2. In addition, voltage-controlled voltage source E2 canconvert compensation voltage signal Vc to voltage signal V2. Theamplitude of voltage signal V2 may be in direct proportion to that ofcompensation voltage signal Vc, and the polarity of voltage signal V2can be same as that of compensation voltage signal Vc. For example,voltage signal Vc can be arranged to be K*Vc, where K is between 0 and1.

In the example of FIG. 3, voltage-controlled voltage source E2 can becoupled in series with voltage-controlled voltage source E1. Forexample, the negative terminal of voltage-controlled voltage source E1can connect to the positive terminal of voltage-controlled voltagesource E2. Therefore, voltage signals V1 and V2 may be added to generatean addition signal to control gate voltage GATE of power transistor Q.In another example, voltage-controlled voltage source E1 can directlyreceive compensation voltage signal Vc. The sum of compensation voltagesignal Vc and voltage signal V1 may be configured to control the gatevoltage GATE of power transistor Q. The waveform of compensation signalVc can generally be shaped as a steady and straight line. Also, drivingvoltage GATE may have a similar waveform to that of voltage signal V1.

In operation, power transistor Q can operate in a linear mode, and thecurrent flowing through power transistor Q may be controlled by drivingvoltage GATE. As shown in FIG. 4, the waveforms of sensing current ISENand driving voltage GATE are the same and have an opposite variation tothat of bus voltage VBUS. When bus voltage VBUS is less than loadvoltage V_(LED), no current may flow through the LED load. Also, whenbus voltage VBUS is larger than load voltage V_(LED), current flowingthrough the LED load can be controlled by control signal regulationcircuit 22.

As shown in FIG. 4, when bus voltage VBUS is greater than a givenrequirement, the current flowing through power transistor Q can becontrolled to be lower, in order to decrease the on-resistance powerloss. Although the power factor may be decreased in this implementation,voltage signal V1 can be adjusted by adjusting the ratio betweenresistors R1 and R2, in order to adjust the LED current to achieve abalance between higher efficiency and power factor correction. In thisway, when the variation of voltage signal V1 is controlled to beconsistent with bus voltage VBUS, the variation of current flowingthrough power transistor Q can be consistent with bus voltage VBUS.

Referring now to FIG. 5, shown is a schematic block diagram of a thirdexample linear LED driving circuit, in accordance with embodiments ofthe present invention. Also referring to FIG. 6, shown is a waveformdiagram of example operation of the linear LED driving circuit of FIG.5, in accordance with embodiments of the present invention. As comparedto the above examples, a third voltage conversion circuit (e.g.,voltage-controlled voltage source E3) and error amplifier EA1 are addedin this example in control signal regulation circuit 22. Sensing voltagesignal Vs can be converted to voltage signal V3 by voltage-controlledvoltage source E3. The amplitude of voltage signal V3 can be in directproportion with that of sensing voltage signal Vs, and the polarity ofvoltage signal V3 may be consistent with that of sensing voltage signalVs.

The non-inverting input terminal of error amplifier EA1 can receive sumsignal VA by adding compensation signal Vc to voltage signal V1, or byadding voltage signal V2 to voltage signal V1. The inverting inputterminal of error amplifier EA1 can receive voltage signal V3. Drivingvoltage GATE may be generated at the output terminal of error amplifierEA1 that represents the error between sum signal VA and voltage signalV3. The current flowing through power transistor Q can be controlled bydriving voltage GATE such that voltage signal V3 follows sum signal VA.Therefore, the variation of the current flowing through power transistorQ can be opposite to that of bus voltage VBUS. In addition, the averagevalue of current flowing through LED load may be consistent with anexpected value. A control loop having a faster response can be formed byerror amplifier EA1 and the third voltage conversion circuit, such thatthe current flowing through power transistor Q can be quickly regulatedaccording to the variation of bus voltage VBUS and the LED current.

By the control of error amplifier EA1, the shape of the waveform of thecurrent flowing through power transistor Q can be consistent with sumsignal VA, in order to avoid potential problems of reduced LED currentcontrol accuracy generated by the variation of parameters (e.g.,temperature) of power transistor Q. In this way, power losses of powertransistor Q can be decreased in order to improve efficiency. Forexample, the first, second, and third voltage conversion circuits can beintegrated into one die (IC). In addition, the voltage dividerresistance network, compensation capacitor, and sensing resistor may beconfigured as discrete devices that form peripheral circuits of the die.

In particular embodiments, control method for a linear LED drivingcircuit including a power transistor can include rectifying analternating voltage signal to generate a sinusoidal half wave directvoltage signal configured as a driving voltage to an LED load. Themethod can also include generating a first voltage signal (e.g., V1)representing the sinusoidal half wave direct voltage signal (e.g.,VBUS). The method can also include generating a driving voltage signal(e.g., GATE) in accordance with the first voltage signal and acompensation voltage signal (e.g., Vc) representing an error between thecurrent flowing through the power transistor (e.g., Q) and an expectedvalue to control current flowing through the power transistor. Themethod can also include controlling the variation of the current flowingthrough the power transistor to be consistent with, or opposite to, thatof the sinusoidal half wave direct voltage signal in one half of powerfrequency period.

For example, the variation of the current flowing through the powertransistor can be controlled to be opposite to that of the sinusoidalhalf wave direct voltage signal in one half of power frequency period.For example, the sinusoidal half wave direct voltage signal is convertedto a first voltage signal with opposite polarity to the sinusoidal halfwave direct voltage signal. For example, the first voltage signal andthe compensation voltage signal can be added to form a sum signal (e.g.,VA) configured as the driving voltage. For example, the compensationvoltage signal can be converted to a second compensation signal indirect proportion with the compensation voltage signal.

For example, the first voltage signal and the second compensation signalcan be added to form a sum signal configured as the driving voltage. Forexample, the sensing voltage signal is converted to a second voltagesignal in direct proportion with the sensing voltage signal. The errorbetween the sum signal and the second voltage signal can be calculatedamplified to form the driving voltage signal. For example, the LEDdriving circuit can include an output capacitor (e.g., Co) coupled inparallel with the LED load, which is coupled in series with the powertransistor.

In particular embodiments, a control method for a linear LED drivingcircuit including a power transistor coupled in series with an LED loadcan include controlling a driving voltage signal of the power transistorto vary with a rectifier output voltage of sinusoidal half waveform, andcontrolling the variation of a current flowing through the powertransistor to be consistent with, or opposite to, the voltage across thepower transistor to decrease the power loss and to improve efficiency.The control method can also include controlling the driving voltage ofpower transistor to make an average value of the current flowing throughthe power transistor to be consistent with an expected value. Forexample, the driving voltage of the power transistor may be controlledin accordance with the rectifier output voltage and compensation voltagesignal representing an error between the average value of the currentflowing through the power transistor and the expected value.

For example, a first voltage signal representing the rectifier outputvoltage can be generated, and a second voltage signal representing thecompensation voltage signal may be generated. The driving voltage may begenerated in accordance with the first and second voltage signals.Alternatively, a first voltage signal representing the rectifier outputvoltage can be generated, and the driving voltage is generated inaccordance with the first voltage signal and the compensation voltagesignal. For example, the amplitude of the first voltage signal can be indirect proportion with the rectifier output voltage, and the polarity offirst voltage signal is consistent with or opposite to the rectifieroutput voltage. The driving voltage may be generated by adding the firstvoltage signal to second voltage signal. Alternatively, the drivingvoltage may be generated by adding the first voltage signal to thecompensation voltage signal.

For example, the amplitude of the first voltage signal can be in directproportion with the rectifier output voltage, and the polarity of firstvoltage signal may be consistent with or opposite to the rectifieroutput voltage. The generating the driving voltage can include addingthe first voltage signal to the second voltage signal to generate a sumsignal, generating a third voltage signal in direct proportion with thecurrent flowing through the power transistor, and generating the drivingvoltage in accordance with an error between the sum signal and the thirdvoltage signal (e.g., V3). In another example, generating the drivingvoltage can include adding the first voltage signal to compensationvoltage signal to generate a sum signal, generating a third voltagesignal in direct proportion with the current flowing through the powertransistor, and generating the driving voltage in accordance with anerror between the sum signal and the third voltage signal.

In this way the variation of the current flowing through the powertransistor can be controlled to be consistent with or opposite to thevoltage across the power transistor by controlling the driving voltageof the power transistor to vary with the rectifier output voltage.Therefore, the variation of current flowing through the power transistorcan be consistent with or opposite to the rectifier output voltage. Whenopposite, the current may be lower while the voltage across the powertransistor is greater, in order to decrease the power loss of powertransistor. When consistent with, the power factor of the LED drivingcircuit may be improved.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to particularuse(s) contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

1. A control circuit for a light-emitting diode (LED) driving circuitconfigured to drive an LED load, the control circuit comprising: a) arectifier circuit configured to convert an input voltage to a rectifieroutput voltage; and b) a control signal regulation circuit configured tocontrol a driving voltage of a power transistor coupled in series withsaid LED load to vary with said rectifier output voltage to control thevariation of a current flowing through said power transistor to beopposite to that of said rectifier output voltage to decrease a powerloss of said power transistor.
 2. The control circuit of claim 1,wherein said control signal regulation circuit is configured to controlsaid driving voltage to control an average value of said current flowingthrough said power transistor to be consistent with an expected value.3. The control circuit of claim 2, wherein said control signalregulation circuit is configured to control said driving voltage inaccordance with said rectifier output voltage and a compensation voltagesignal that represents an error between said average value and saidexpected value.
 4. The control circuit of claim 3, wherein said controlsignal regulation circuit is configured to: a) generate a first voltagesignal having a polarity opposite to said rectifier output voltage; b)generate a second voltage signal that represents said compensationvoltage signal; and c) generate said driving voltage in accordance withsaid first and second voltage signals, or in accordance with said firstvoltage signal and said compensation voltage signal.
 5. The controlcircuit of claim 4, wherein said control signal regulation circuitcomprises: a) a voltage divider resistance network configured to dividesaid rectifier output voltage to generate a first voltage dividersignal; and b) a first voltage conversion circuit configured to generatesaid first voltage signal with an absolute value that is in directproportion to said rectifier output voltage by converting said firstvoltage divider signal.
 6. The control circuit of claim 5, wherein saidcontrol signal regulation circuit comprises: a) a second voltageconversion circuit coupled in series with said first conversion circuit,and being configured to generate a second voltage signal in directproportion with said compensation voltage signal; and b) wherein a sumsignal is generated at one terminal of a series connection of said firstand second conversion circuits, or by adding said first voltage signalto said compensation voltage signal.
 7. The control circuit of claim 6,wherein said sum signal is configured as said driving voltage to controlsaid power transistor.
 8. The control circuit of claim 6, wherein saidcontrol signal generation circuit comprises: a) a third voltageconversion circuit configured to convert a sensing voltage signal thatrepresents said current flowing through said power transistor to a thirdvoltage signal in direct proportion with said sensing voltage signal;and b) a first error amplifier configured to generate said drivingvoltage in accordance with said sum signal and said third voltagesignal.
 9. The control circuit of claims 1, wherein said control circuitcomprises a feedback comparison circuit configured to generate saidcompensation voltage signal in accordance with a sensing voltage signalthat represents said current flowing through said power transistor, anda reference voltage signal that represents an expected value of acurrent flowing through said LED load.
 10. The control circuit of claim9, wherein said feedback comparison circuit comprises: a) a second erroramplifier configured to generate said compensation voltage signal inaccordance with said sensing voltage signal and said reference voltagesignal; and b) a compensation circuit coupled between an output of saidsecond error amplifier and ground.
 11. The control circuit of claim 1,wherein said rectifier output voltage is configured as a sinusoidal halfwave direct voltage signal.
 12. The control circuit of claim 1, whereinan anode of said LED load is connected to said rectifier output voltage,a cathode of said LED load is connected to a drain of said powertransistor, a source of said power transistor is connected to a firstterminal of a sensing resistor, and a second terminal of said sensingresistor is connected to ground.
 13. A control method for alight-emitting diode (LED) driving circuit configured to drive an LEDload, the method comprising: a) converting, by rectifier circuit, aninput voltage to a rectifier output voltage; and b) controlling, by acontrol signal regulation circuit, a driving voltage of a powertransistor coupled in series with said LED load to vary with saidrectifier output voltage to control the variation of a current flowingthrough said power transistor to be opposite to that of said rectifieroutput voltage to decrease a power loss of said power transistor. 14.The method of claim 13, further comprising controlling said drivingvoltage of said power transistor to control an average value of saidcurrent flowing through said power transistor to be consistent with anexpected value.
 15. The method of claim 14, further comprisingcontrolling said driving voltage in accordance with said rectifieroutput voltage and a compensation voltage signal representing an errorbetween said average value and said expected value.
 16. The method ofclaim 15, wherein said controlling said driving voltage comprises: a)generating a first voltage signal having a polarity opposite to saidrectifier output voltage; b) generating a second voltage signalrepresenting said compensation voltage signal; and c) generating saiddriving voltage in accordance with said first and second voltagesignals, or in accordance with said first voltage signal and saidcompensation voltage signal.
 17. The method of claim 16, wherein theabsolute value of said first voltage signal is in direct proportion withsaid rectifier output voltage, and said generating said driving voltagecomprises adding said first voltage signal to said second voltagesignal.
 18. The method of claim 16, wherein the absolute value of saidfirst voltage signal is in direct proportion with said rectifier outputvoltage, and said generating said driving voltage comprises adding saidfirst voltage signal to said compensation voltage signal.
 19. The methodof claim 16, wherein said generating said driving voltage comprises: a)adding said first voltage signal to said second voltage signal togenerate a sum signal; b) sensing said current flowing through saidpower transistor to generate a sensing voltage signal; c) convertingsaid sensing voltage signal to generate a third voltage signal in directproportion with said sensing voltage signal; and d) generating saiddriving voltage in accordance with an error between said sum signal andsaid third voltage signal.
 20. The method of claim 16, wherein saidgenerating said driving voltage comprises: a) adding said first voltagesignal to said compensation voltage signal to generate a sum signal; b)sensing said current flowing through said power transistor to generate asensing voltage signal; c) converting said sensing voltage signal togenerate a third voltage signal in direct proportion with said sensingvoltage signal; and d) generating said driving voltage in accordancewith an error between said sum signal and said third voltage signal. 21.A control circuit for a light-emitting diode (LED) driving circuit todrive an LED load, the control circuit comprising: a) a rectifiercircuit configured to convert an input voltage to a rectifier outputvoltage; and b) a control signal regulation circuit configured tocontrol a driving voltage of a power transistor to vary with saidrectifier output voltage to control the variation of a current flowingthrough said power transistor to be consistent with that of saidrectifier output voltage to improve a power factor of said LED drivingcircuit.
 22. The control circuit of claim 21, wherein said controlsignal regulation circuit is configured to control said driving voltagein accordance with said rectifier output voltage and a compensationvoltage signal that represents an error between an average value of saidcurrent flowing through said power transistor and an expected value. 23.The control circuit of claim 22, wherein said driving voltage iscontrolled in accordance with error information between a sensingvoltage signal that represents said current flowing through said powertransistor, and a sum of said compensation voltage signal and a firstvoltage signal that is in direct proportion with said rectifier outputvoltage.
 24. The control circuit of claim 3, wherein said control signalregulation circuit is configured to control said driving voltage inaccordance with said rectifier output voltage, said compensation voltagesignal, and a sensing voltage signal that represents said currentflowing through said power transistor.
 25. The control circuit of claim24, wherein said driving voltage is controlled in accordance with errorinformation between said sensing voltage signal and a sum of saidrectifier output voltage and said compensation voltage signal.
 26. Acontrol method for a light-emitting diode (LED) driving circuit to drivean LED load, the method comprising: a) converting, by a rectifiercircuit, an input voltage to a rectifier output voltage; and b)controlling, by said control signal regulation, a driving voltage of apower transistor to vary with said rectifier output voltage to controlthe variation of a current flowing through said power transistor to beconsistent with that of said rectifier output voltage to improve a powerfactor of said LED driving circuit.
 27. The method of claim 26, furthercomprising controlling said driving voltage in accordance with saidrectifier output voltage and a compensation voltage signal representingan error between an expected value and an average value of said currentflowing through said power transistor.
 28. The method of claim 27,wherein said driving voltage is controlled in accordance with errorinformation between a sensing voltage signal that represents saidcurrent flowing through said power transistor, and a sum of saidcompensation voltage signal and a first voltage signal that is in directproportion with said rectifier output voltage.